Our society is increasingly dependent upon larger and more complex networks of civil infrastructure that are costly to maintain. These infrastructures, especially those for transportation, are deteriorating due to factors such as corrosion of the steel reinforcement and high continuous vibration. This type of degradation has motivated research activity on distributed wireless sensors that enable low-cost, near continuous, and near real-time, non-destructive monitoring.
In order to efficiently provide such monitoring capabilities, it is preferred to have the sensor node embedded within the structure in direct proximity of the physical or chemical activity of interest. Embedding the sensor node requires the device to be passive or recharged externally to extend the lifetime because of the difficult accessibility, and to have a compact geometry to avoid compromising the structural integrity and to facilitate the deployment of the node into the preferred location. In order to communicate with embedded sensor nodes over a long distance and from a convenient location, the sensing device should require low radio frequency (RF) activation power and be energy efficient.
Another challenge associated with passive embedded wireless monitoring is remote channel calibration. When the sensor node is deployed in a structure, it is of concern to periodically calibrate the propagation channel (between the interrogator and the node) to obtain a proper reading from the sensor and confirm that the node is functioning properly. Although such calibration can be obtained by placing a reference node in close proximity to the sensor node, the requirement for an additional node to be placed close to each sensing node is undesirable.
From the above discussion, it can be appreciated that it would be desirable to have alternative apparatus that enables remote channel calibration of an embedded sensor node.